Significant improvement in early post-transplant survival rates has been achieved over the last decade for recipients of all types of allografts. Nevertheless, the need for further refinement of currently available nonspecific immunosuppression is emphasized by the morbidity and the annual attrition rate of at least 3-5% which are observed in chronically suppressed allograft recipients and the complete failure of such therapy for xenogeneic transplantation. The objective of this Program Project is to improve the outcome following organ transplantation by defining the essential conditions for and clarifying the mechanisms involved in donor specific tolerance induction. The ultimate goal is to induce specific and long-lasting inhibition of only those elements of the immune response responsible for allograft or xenograft rejection without the need for long-term administration of immunosuppressive agents. The rationale linking the objectives of the 4 interrelated and even in some human allograft recipients, it should be possible to identify and then reproducibly provide the immunologic perturbations which lead to this state. Preliminary observations have confirmed that one approach, employing mixed chimerism, can be successfully extended from murine models to non-human primate allo and concordant xenograft recipients. Our hypothesis is that definition of an approach to tolerance induction can be applied clinically will not only improve the long-term results of allotransplantation, but also provide a means of utilizing xenogeneic donors. The specific aims are to characterize, in a continuum of models including mice selected for specific MHC incompatibilities or genetically engineered defects, partially inbred swine, and non-human primates, the mechanisms of action of conditioning regimens, with or without donor bone marrow transplantation, that induce tolerance and the state of reactivity of anergy that results from their application. The methodology of the studies includes genetic manipulation, renal or cardiac allo and xenografts in large animal models, thymus transplantation, and flow cytometry and functional analyses of peripheral blood and graft infiltrating cells. Subsequent translation of these developments to clinical protocols for treatment of allograft recipients will be of immediate relevance and should contribute to a reduction in the costs and morbidity associated with transplantation and should ultimately provide a realistic approach to even discordant xenotransplantation.